Chlamydia is a pathogenic bacterium with a significant impact on public health. In 2006, more than a million chlamydial infections were reported to the CDC making it the most commonly reported infectious disease. The ability of this organism to cause disease is related to its unusual developmental cycle, which takes place inside a human cell. Our long-term objective is to understand how this pathogen controls the expression of its genes during the developmental cycle so that it can grow and replicate. Our central hypothesis is that the three temporal classes of early, mid and late chlamydial genes are coordinately regulated at the transcriptional level by distinct mechanisms. Aim 1 will determine if the higher levels of chlamydial DNA supercoiling measured in midcycle are used as a general mechanism to upregulate mid genes. Aim 2 will investigate if early genes are selectively expressed at the start of the infection because they are resistant to an inhibitor that prevents transcription of later temporal classes of genes. Aim 3 will examine two regulators that repress late genes to prevent their premature expression. Successful completion of these studies will help us to understand how Chlamydia controls the programmed expression of its genes. These findings may lead to novel therapeutic strategies for treating chlamydial infections by interrupting the developmental cycle.